Method and apparatus for making geological explorations



May 16, 1939. H. PRESCOTT Filed J 25, 1935 MW im INVENTOR Ham/a 1QPrescott ATTORN EY METHOD AND APPARATUS FOR MAKING GEOLOGICALEXPLORATIONS 6 Sheets-Sheet l May 16, 1939. H. R. PRESCOTT 2,153,198

METHOD AND APPARATUS FOR MAKING GEOLOGICAL EXPLORATIONS Filed July 25,1935 6 SheetsSheet 2 jeisma hane v -mmm Amp/H de Paco/derl 1 6'4 78 A 70fig 66 79 77 75 4,0 4/ 4 q 47 a a E E u u m a u D I: a a u n m D I: u DE D I: D fill mlfilfill llllllll lllAil lllll Qlllllll ||||ll J A A 7 l11 I 4 9? 95 95 J97; 25' l 1 l l 1 F M V? 1 I? u D m I: u U U a D U I: DD D u u D I: III I: D D u D I:

INVENTOR Harv/a R Fresco tt ATTORNEY y 15, 1939- H. R. PRESCOTT2,158,198

METHOD AND APPARATUS FOR MAKING GEOLOGICAL EXPLORATIONS Filed July 25,1935 v 6 Sheets-Sheet 5 T/vcei No.2

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METHOD AND APPARATUS FOR MAKING GEOLOGICAL EXPLORATIONS INVENTQR HaroldE. Prescott ATTORN EY May 16, 1939.

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ATTORNEY Patented May is, 1939 um'rao s'm'ras PATENT orrica METHOD ANDAPPARATUS FOR G GEOLOGICAL EXPLORATIONS Harold It. Prescott, Ponca City,Okla, asslgnor to Continental Oil Company, Ponca City, Okla, acorporation of Delaware My invention relates to a method and apparatusfor making geological explorations and more particularly to a method andapparatus for automatically limiting electric manifestations in sympathywith'earth vibrations.

In application Serial No. 721,541, in which Frank L. Searcy and myselfare joint inventors, a device for automatically limiting the amplitudeof recorded earth vibrations independent of the amplitude of vibrationsreceived by a seismophone is shown. The method and apparatus inapplication Serial No. 721,541, functions well when the seismophones orreceptor stations are well removed from the point of origin of theelastic waves. When, however, seismophones are placed close to the shotpoints or points of origin of the seismic waves, the apparatus .shown inapplication Serial No. 721,541 fails to accomplish the desired results.

In the accompanying drawings which form part of the instantspecification and are to be read in conjunction therewith and in whichlike reference numerals are used to indicate like parts in the variousviews,

Figure 1 is a diagrammatic cross sectional view of a section of theearths surface showing some paths taken by vibrations generated near theearths surface.

Figure 2 is a schematic view of the form of apparatus shown inapplication Serial No. 721,541.

Figure 3 is a conventionalized view of a record strip taken with theapparatus shown in application Serial No. 721,541.

Figure 4 is a graph explanatory of the action of the apparatus shown inapplication Serial No. 721,541 under conditions in which seismophoneswere well removed from the shot points.

Figure 5 is a graph explanatory of the action of the apparatus shown inSerial No. 721,541 under conditions in which the seismophones are placedclose to the shot points.

Figure 6 is a diagrammatic view of one form of device capable ofcarrying out the process and embodying the apparatus of my invention.

Figure 7 is a graph explanatory of the apparatus shown in Figure 6.

Figure 8 is a diagrammatic view of another form of apparatus capable ofcarrying out the method and showing another embodiment of my invention.

Figure 9 is a graph showing the relationship between amplification andthe amount of positive potential for one of the tubes shown in Figure 8.

Referring now to Figure 1 in which I repretheir respective outputs torecorder l3. vibrations generated at 8 travel out in all direc- 20 sentsthe weathered or unconsolidated layer of the earths surface and layer 2a consolidated layer of the earths crust, layer 3 is a formation ofdifferent characteristics than layer 2. Layer 4 is a layer of the samegeneral characteristics 5 as layer 2. Layer 5 has the same generalcharacteristics as layer 3' and layer 6 is similar in character tolayer}. It is desirous to position the point of origin of the vibrationsin a consolidated layer. To this end, a drill hole 1 is 10 made throughthe weathered layer l to the layer 2. A charge of explosive 8 ispositioned in the bottom of the drill hole I. I3 represents a recordingdevice, conductors 9 and in recording the instant of the origin of theearth vibrations. H 15 and I2 represent seismophones connected bysuitable conductors to respective amplifying devices it and 55 which, inturn, are adapted to transfer The tions. The direct wave will arrive atthe vibration sensitive devices by approximate paths 8iS--l I, and 8l1l2. Part of the vibrations will pass downwardly t'o strata 3 and aportion will be returned or echoed back to the earths 25 surface. Path8l8lG-H is the approximate path of travel of such vibrations whichtravelto the seismophone ll. Path 8-l9l1--|2 is the approximate path of thevibrations which travel to seismophone l2. The vibration sensi- 30 tiveinstruments H and I2 receive these vibrations and transfer them intosympathetic electrical vibrations which are amplified by respectiveamplifying means I l and I5, and recorded in recorder I3 upon a commonrecord strip which is later removed for analysis.

Weathered layer lot the earths surface is of variable thickness. It willbe noted that the vibrations traveling along path 8l6ll pass throughless thickness of weathering than the 40 vibrations traveling along path8-41-12. If it is assumed that the vibrations reaching point l6 were ofamplitude equal to those reaching point H, the vibrations received at II would be different than those received at l2, due to the variation 45in the thickness of the weathered layer I.

Similarly, other waves will penetrate to the deeper lying strata 5following paths 820--I6 II and 8-'-2ll'|l2 respectively.

Referring now to Figure 2, suspended from a 50 for brevity, we willrefer to as a seismophone. As the vibrations are received. the frame 38will move in phase with the vibrations. The electromagnet 32, because ofits mass, will tend to remain stationary in place, varying the magneticflux flowing through core 33 and thus inducing an electromotive force incoils 34 in phase with the vibrations. The conductor 35 impresses thegenerated voltages upon the grid 38 of thermionic tube 81. Plate currentflows from plate battery 38 through conductor 38 through primary 48 oftransformer 4| to the plate 42 of thetube 31, to thefllament 43,throughconductor 44 to the negative terminal 45 of the plate battery 38.Filament battery 48' is provided to heat the filament- 43. Variations ofvoltage m the grid 38 will cause vibrations in the plate current insympathy therewith. The variations in the plate. current i are picked upby the secondary coil 41 of transformer 4| and impressed'upon the grid48 of tube .49. Plate current will flow from plate batfies the voltageand delivers a pulsating direct current to a filter. The filter sectionconsists of a capacity 18 and a resistance H, and serves to .smooth thedirect current proportional to the average voltage delivered by thetransformer coil 8| to the rectifying tube'65. As a result of this,

a direct current voltage is impressed by conductor 12, conductor 13,conductor 35, upon the. grid 38 of vacuum tube 31,

It is well known in the art that the sensitivity of thermionic tubes, oramount of amplification, may be controlled by the adjustment of thedirect current voltage upon the grid. The voltage supplied to grid 36from the rectifying tube 65 and filter will control the amplificationvof vacuum tube 31. By proper variations of transformer winding 8i,vacuum tube rectifier 65, resistance H, and capacity 10 of the filter,the amplification of tube 31 "can be controlled to any predeterminedlimitations, and the energy delivered to the recording unit will be heldto a constant amount regardless of the interrupted voltage delivered bythe seismophone.

A coil 14, inductively coupled to coil 52, delivers voltage to coil I5,wound around core 16. Pivoted adjacent core 16 is an armature Tiprovided with a spring 18. The armature 'll carries a mirror 18 uponwhich is focused by lens 88 a beam of light from incandescent light 8|;A battery 82 supplies current for the light 8|. The reflected beam frommirror 19 passes along the path 83 to a photographic film 84 which ismoved rapidly by motor 85. As the plate current of tube 48 varies, thecoil '14 of the transformer 53 will deliver varying voltages to coil 15,thus causing a varying current therein. This, in turn, will induce amagnetic field in phase with the vibrations being received in core 18,attracting the armature 11 more or less strongly, making a recor uponthe photographic film 84.

When the seismophones are placed at a considerable distance from theshot, that is, when the distance from 8 to l I in Figure 1 is great, the

amplitude of the waves along the path 8|8-|| in Figure 1 is onlyslightly larger than that of the early reflected waves which followpaths 8-l8-ll and 8-28l|. In this case, the'apparatus shown at Figure "2will workwell and receive a record such as shown in Figure 3. Theobjects of the invention of application Serial No. 721,541 will befulfilled, namely, the true beginning of the direct wave will be at fullamplitude on,the record and the following waves will be controlled orgoverned to a uniform amplitude.

Referringnowto Figure 4, trace l is a graph of the amplitude received atseismophone ||,Lplotted at the instant of origin of the seismic waves at8. For-a short time following the detonation, no amplitude of motionwould exist at the seismophone station. When the direct wave along path8-t8--Il reaches the seismophone station II, the amplitude will reachthe'maximum, as shown by point E. The amplitude of reflected wave 8--l8-|l is represented by point A. The amplitude of reflected event 8-28-|is represented by point B. C and D represent the amplitudes of eventsdeeper than, A and B, not shown on Figure 1. As a result of theseamplitudes of motion, there will be developed a floating governingcharge on filter condenser 18 of Figure 2 and the sensitivity of theamplifier is shown by the curve, trace 2. This curve is plotted inpercent of maximum sensitivity= Before the direct wave arrives alongpath 8| |--|8 of Figure 1, the amplifier will be receiving no electricalvoltage from the seismophone, because no motion has taken place and theamplifier is fully sensitive. The first impact E of the direct wavearriving along path 8 -l8|| of Figure 1 will come at maximum sensitivityand the rectifying tube 65 of Figure 2 will receive a voltage from theoutput winding BI and shortly thereafter a floating charge, forgoverning purposes, will be acquired by condenser 18. This governingcharge will reduce the sensitivity of tube 31 as shown by trace 2 ofFigure 4. After the amplitudes of motion have grown smaller, thegoverning charge becomes smaller because the output voltage at 8| isless. Underthese conditions, the amplifier resumes greater sensitivityand approaches full sensitivity as the amplitudes of motion die out.Knowing the amplitude of motion and the sensitivity (traces l and 2 ofFigure 4) we may compute the resulting amplitude which will be recordedby the recording mechanism upon sensier amplitude than the followingreflected waves arriving along paths 8|8|| and 8-2|l||. The highamplitude direct wave causes a large voltage input to the amplifierfromthe seismophone with a'. resulting high voltage charge on condenser18 of Figure 2. The high voltage charge completely blocks tube 31 andmakes it very insensitive with the result that the amplifier will notresume useful governed sensitivity until the high voltage floatingcharge has leaked oil through resistance ll of Figure 2. I have found,in practice that, when the seismophone is placed close to the shot pointand the apparatus shown in Figure 2 is employed, that a record having adead space with no recorded amplitudes for the early geological stratais obtained. The device will record control values of arrivals fromintermediate and deeper strata.

Referring now to Figure which is a graph showing the conditions obtainedwhen seismo-- phones are placed close to the shot point, using theapparatus shown in Figure 2, trace l is the average amplitude receivedat selsmophone ll, plotted in percent of maximum. Zero time is theinstant of detonation of the explosive charge at point 8. As in Figure4, no amplitude of motion will existat selsmophone l l until the directwave arrives along path 8l6l l. amplitude is represented by point E. Theamplitude of reflected event 8|8Il is represented by point A. Theamplitude of reflected event 8-20-ll is represented by point B. C and Dare reflections of events from deeper strata than A and B not shown onFigure 1.

It will be observed that the direct wave arrival is much greater inamplitude than that of the following events. Trace 2 represents thesensitivity of the amplifier and point X shows that the sensitivity isreduced,so much that, during this time interval, a portion of therecorded arrivals are not amplified enough to be useful. The timeconstant of the filter circuit in the amplitude limiter is heldsufliciently low, however, so that the high voltage floating chargeleaks ofi rapidly and the amplifier resumes a useful governedsensitivity sufiiciently soon to produce intermediate and late reflectedevents. The time constant of the resistance-capacity network'may bedecreased by lowering either the value of resistance II or the capacityIll or both, of the circuit shown in Figure 2.

Trace 3 of Figure 5 represents the resulting amplitude reproduced by therecorder on sensitive strip 84. It will be observed that a useless ordead space is shown corresponding to the portion X in which thesensitivity of the amplifier was reduced by the high blocking voltageupon condenser 10. This explains the dead space received upon therecord. This dead space upon the record is undesirable because thereflected waves from shallow geological strata which would occur at thisplace on the record are lost. This is the first disadvantage-of the formof apparatus shown in Figure 2. v

In order to have the amplifier return rapidly to a useful governedsensitivity, the time constant, 7

as has been heretofore pointed out, must be sufficiently low for thehigh voltage initial charge to leak off rapidly. During this portion ofthe curve, the sensitivity is changing so rapidly that amplitudedistortion of the wave train of reflected energy may occur, that is,even though the first cycle of wave motion were equal in amplitude tothe trailer cycles, the trailer cycles will be reproduced with greaterresulting recording amplitude as a result of rapidly increasingsensitivity. This constitutes a disadvantage of the form of apparatusshown in Figure 2.

Even when the small amplitudes exist at the seismophone, such as thoseproduced by the late reflected waves, a small governing charge existsthe motions become extremely low in amplitude.

This may be further seen by trace 2 of Figure 5. In both instances, thesensitivity approaches max- The maximum,

while controlling or limiting all intermediate.

amplitudes to a-convenient value, for analysis.

By permitting the first arrivals to be recorded at maximum sensitivity,an accurate appraisement of the length of time of the directwave toarrive. supplies a means for computing the thickness of the weatheredlayer.

Another object of my invention is to provide a means and method ofcontrolling the amplitude of arrivals, while permitting the recording ofrefiected waves from shallow geological strata.

Another object of my invention is to provide a means and method oflimiting the amplitude of electrical manifestations and of geophysicalvibrations admitting of the use of a larger time constant filter networkto prevent amplitude distortion.

Another object of my invention is to provide a means of geophysicalexploration in which reflected events from shallow and intermediatestrata are recorded at a governed amplitude while permitting the recordof late reflected events from deep lying geological strata to berecorded at maiximum sensitivity of the amplification channe I Anotherobject of my invention is to provide a device for recording geophysicalvibrations at governed amplitudes, which will be operative regardless ofthe location of the seismophone with respect to the point of origin.

Other and further objects of my invention will appear from the followingdescription.

In order to eliminate the dead spot on the record, which occurs when theseismophones are placed close to the shot point and in order to receiveelastic waves from shallow strata, I make use of a circuit network whichI term the upper threshold. In order to receive the late reflectedevents from deep lying strata at the full sensitivity of the amplifier,I make use of a circuit network which I term the lower'threshold.

During the time interval after the direct waves have arrived and beforethe late reflected events from the deep lying stratamccur, the amplifiersensitivity is governed between values set and determined by manualadjustment of the upper and lower thresholds. I, would like to pointout, at this time, that during the time interval occurring after theearly arrivals and before the late arrivals, the amplifierisautomatically governed in sensitivity in the apparatus shown in Figure2, by the floating charge of condenser 70. The dual thresholdadjustments determine the limits for the amplifier sensitivity.

Having two limits within which the sensitivity of the amplifier isgoverned, the arrangement permits of a more suitable selection of thetime constant of the filter circuit network so that amplitude distortionwill be eliminated as will hereinafter be more fully pointed out.

Let us consider first, the case in which seismophone II is close to theshot point 8. When an explosive is detonated at the shot point 8, thedirect wave arriving along path 8-l6--|l is much greater in amplitudethan the waves arriving along path 8-i8ll or by path 8-40- H. Theamplitude relationship is shown by trace I of Figure "I in which thedirect wave appears at point E, the arrival along path 8-48- II by pointA, and the arrival along path 8' 20-I I by point B. Points C and Drepresent later arrivals from deeper lying strata not shown in Figure 1.

The seismophone of Figure 6 receives the voltage occasioned by the firstdirect wave arrival represented by point E of Figure 7, and delivers tothe input of the amplifier a voltage variation in phase with. themotions of the earth at the seismophone position. The first vacuum tube31 of Figure 6 receives this voltage variation and delivers it throughtube 49 to the primary winding 52 of the transformer. The secondarywinding Glyof transformer 53 delivers a voltage variation to therectifier tubewhich places a floating charge on filter condenser IOI.Let us make reference at this time to the lower threshold network ofFigure 6. The action of winding 6| is to place alternately a positiveand then a negative voltage on a lead wire 62, inphase with the motionsof the earth as they move upwardly and downwardly at the seismophoneposition. When the grid plate combination .I03 receives positivepotential, the rectifier tube I04 passes current to the filter networkI05 and a floating charge is built up on condenser IOI. When the voltagebecomes negative the tube does not pass current. It will be observedthat, if the adjustable arm I09 were moved downwardly so that resistanceE102 were bypassed, the arrangement wouldbe that shown in Figure 2 andthe lower threshold would not function to place a bias on lead wire 62,and the function of the lower threshold would be eliminated.

The battery I01 in the lower threshold has a negative terminal towardthe grid plate combination E03 of tube I04. As the arm I09 is movedupwardly over resistance I02 a negative threshold bias is placed uponthe grid plate combination I03 of the rectifier tube E04, the amount ofthe bias depending upon the setting of arm I09.

When the winding GI of the transformer 53 places a positive voltage onlead wire 62 equal to the threshold bias of the lower threshold, thethreshold bias being opposite in polarity will nullify the positivevoltage developed by the winding and no positive potential will bedelivered to the rectifier tube I04. When the winding 6| has inducedtherein a greater positive voltage than the opposing threshold value,the rectifier tube I04 receives a positive resulting potential anddelivers a. governing floating charge to the filter network I05. It willbe appreciated that,.by proper adjustment of the arm I09, all of thesmall amplitude arrivals from deep geological strata may be reproducedwith full sensitivity of the amplifier, while the arrangement willretain the desirable feature of limiting the amplitude of arrivals fromintermediate strata. This followsv because the threshold bias may besets to nullify the positive potential delivered by winding 6| for thesmall amplitude arrivals from deeper. strata. It will be observed thatthe -so as to bypass all voltage above a certain amount; asdetermined bythe setting of arm H3. If battery H2 were one of high voltage and thearm H3 moved upwardly, the voltage delivered by winding 6| to conductor62 would not be sufiiciently high to be bypassed by the upper thresholdnetwork because the opposing potential of battery H2 would prevent this.

When battery H2 is of voltage within the range developed by winding 0iand the adjustable arm H3 is moved downwardlyofi resistance H4, therewill be no opposing potential placed in the upper threshold network andall positive voltage which is developed on conductor II9II5 will bebypassed by the tube III. This follows because the grid platecombination I20 of the rectifier tube III will pass current as soon aspositive potential occurs upon the grid plate combination. It will beobserved that, when the adjustable arm H3 is set downwardly at aposition oil of resistance H4, no current is passed by the rectifiertube I04 to the filter network I05, because the impedance of therectifier filter network I05 is high in comparison to the upperthreshold network. The upper threshold, therefore, acts as a by-pass forall voltages above the value determined by arm H3. In the positionreferred to, namely, when arm H3 is moved downwardly ofi' resistance M4,the amplifier will fail to govern or limit amplitudes because nofloating, governing, or limiting charge will be acquired'by thecondenser IOI.

If we have determined the amount of the positive peak voltage deliveredto conductor 62 by the winding SI for the arrival A of Figure 7,

we may now set the arm H3 at some intermediate point such as shown inFigure 6 so that the positive threshold bias in the upper thresholdnetwork is equal to this positive peak voltage. With this adjustment,let us consider the action of the amplifier when the seismophone isplaced near the shot point and receives a. large amplitude direct wavefor the first arrival. This is the situation shown in Figure 7. Theupper threshold bias has been adjusted to a value equal to that of thepositive peak voltage delivered to conductor 62 as shown by amplitude Aon trace I. Now, when a greater amplitude of motion occurs thanrepresented by the value of point A on trace I of Figure 7, a greatervoltage will be delivered by the winding GI and the upper threshold willbypass he excess voltage above that to which the threshold has been set.In other words, the entire action would be as follows: The direct wave8-I6'--I I, represented by point E of Figure 7, would engage theseismophone and the first peak of motion would occur at full output onthe-output transformer. This would deliver a large voltage on winding GIand the excess voltage over the threshold would be bypassed by tube III.work, it will be observed, will prevent the total charge acquired by thecondenser IOI of the filter network I05, from being greater in amountthan the value of the voltage determined by arm H3. If the arm isproperly set at the value of the amplitude of point A in Figure 7, thegoveming charge is just sufficient to govern the first useful geologicalstrata arrival A and all excess positive voltage caused by the firstarrival E will The upper threshold netbe bypassed by the upperthreshold. This network of the upper threshold prevents the dead spotsuch as shown in Figure 5, thus permitting the early arrival A to bereproduced at a governed' or limited amplitude suitable for analysis.while in no way preventing the reproduction of intermediate amplitudesat governed or limited amplitudes.

Trace 2 of Figure '7 represents the resulting sensitivity of anamplifier with the upper threshold in use, the sensitivity occurringfrom an amplitude represented by trace I.

Trace 3 of Figure 7 shows the amplitude on the record. I have conductedactual operations in the field and have been able to limit all motionsimmediately following the direct wave or first arrival to suitablevalues with seismophones located as close as 75 feet from the shotpoint, It will be observed from trace 2 of Figure '7 that, when theamplitude of motion becomes smaller, the amplifier reaches one hundredpercent sensitivity due to the useful action of the lower thresholdnetwork.

It will also be observed that it will not be necessary to use a low timeconstant in the filter circuit network because the sensitivity need notincrease as rapidly as it would in the case of Figure 5.

The arrangement I have described is simple and practical in actual useand does not require diflicult adjustment and constant attention. Thedesired amplitude of the required'waves can be readily ascertained fromthe physical dimensions of the photographic record and the particularamplitude which is deemed convenient for analysis on the particular sizeof record. The lower threshold value is set by arm I09 so that thedesired amplitude is obtained upon the photographic record. This value,once set normally, does not require readjustment for the particularapparatus involved. When a new area is explored, the time constant ofthe filter network I05, that is the limiting governing filter, isadjusted by setting arm I22 upon resistance IIO.

This time constant is adjusted so that the rate threshold and is made bymoving arm II 3 .over

resistance IIH. This adjustment is made at a 1 point where the upperthreshold will bypass the first arrivals or direct waves which areappreciably higher in amplitude than the early useful reflected waveswhen the seismophone is near the shot point. When this adjustment ismade, the seismophone may be removed from the shot point and the entirenetwork will govern quite satisfactorily.

After the three adjustments just referred to, namely, those of arm I09,arm I22, and arm H3, have been made, which adjustments can beaccomplished very easily with one .or two experimental shots in a newarea, the. entire area may be explored with the assurance that a highpercentage of all records taken will show all arrivals from usefulgeological strata including the early and late arrivals of usableamplitudes.

The arrangement shown in Figure 6 is merely one arrangement of myinvention and is a Simple circuit which I have shown by way ofillustration for use in explaining my invention. It will be understoodby those skilled in the art that there are many variations which may beused such as the employment of the multiple element vaguum tube foraccomplishing both the functions of rectifier tube I04 and rectifiertube III. e

In actual practice, I prefer to use an arrange ment such as shown inFigure 8. This arrangement is more flexible in design. I employ iii--directly heated vacuum tubes and full wave rectification to supplyvoltage to the filter condenser for the floating charge. l

In Figure 8, I show a. modification of an upper threshold arrangement inwhich the cathode 202 of tube 208 is indirectly heated by heater coil20!. As understood by those skilled in the art, when-the cathode 202becomes heated, it will emit electrons and current will pass from plate203' to the cathode 202, independent of the heating coil 20I. As in anordinary vacuum tube, when the grid .206 receives positive potential, agreater current will fiow from the plate 203 to the cathode 202 and whenthe grid 206 receives a negative potential, less plate current willflow.

In the ordinary vacuum tube, the maximum amplification is obtained fromthe tube by placing a negative potential of optimum value upon the gridreturn circuit. In the arrangement shown in Figure 8, the cathode togrid bias is positive an amount determined by arm 205 and battery 204The relationship between amplification and the amount of bias for one ofthe indirectly heated cathode tubes used in the arrangement shown inFigure 8 is shown in Figure 9, in which the bias is plotted againstpercent maximum amplification. From Figure 9 it can be readily seen thatthe tube used reaches its maximum sensitivity when the arm 205 isadjusted so as to have a bias of about three volts.

If, now, the bias upon conductor 201 of Figure 8, were increased, thiswill perform substantial-ly the same result as increasing the cathodebias one volt positively. .Under these circumstances, with one voltnegative bias on conductor 201, the tube 208 will be operating atreduced sensitivity, namely at the point of a Figure 9. If the bias onconductor 20'! of Figure 8 were increased positively one volt, the netresult would be equivalent to lowering the cathodes positive bias onevolt and, under these circumstances, the tube 208 will be operating at areduced sensitivity as at point b of the curve shown in' Figure 9.

From these considerations, it is apparent that either a positive ornegative floating charge upon filter condensers 209 and 2I0 will governthe sensitivity but only if the arm 205 is such that the amplificationwill be a maximum if there is no governing charge of the filtercondensers 209 and 2).

In some of the arrangements I have used, I have employed a circuitnetwork delivering a negative potential to the governing filtercondensers, but for simplicity of network, I prefer to supply a positivefloating charge such as is supplied in the arrangement shown in Figure8.

Full wave rectification is provided by the center tapped winding 2| I.One conductor 2| 2 from one side of the winding 2 goes to one anode 2Mof the full wave rectifier tube. The second conductor 2I3 from theopposite side of winding 2 leads to the second anode 2I5 of the full 7ppass current when a negative potential is impressed on the anode. As aresult of the center tapped winding 2| I and the connections 2l2 and2|3, when the potential of conductor 2l2 becomes positive, anode 2 willpass current to the cathode 2|! and filter network. When conductor 1213becomes positive, anode 2l5 will pass.

current to the cathode and filter network. This full rectificationprovides a floating charge on the filter condensers 209 and 2H! freer ofrapid variations and makes it easier to render the floattion areinterposed between the first tube 208 and v the last tube 220 of theamplifier. This prevents distortion when a very large governing chargeis acquired by the filter system. This distortion may be explained asfollows: when one governing charge is on the filter system, the firsttube is fully sensitive and only a small voltage variation is requiredon the first grid network to produce usable amplitudes on recorder strip84. When a large governing charge is placed on the filter netv work, thefirst tube becomes quite' insensitive and, in order that the recordreceive usable amplitudes, the voltage (as for example that gen eratedby the arrival of reflections from shallow geological horizons) will begreat and the tube will distort because the voltage variation on thegrid will pass the limits where the plate current variation will belinear with respect to grid vari ations. This latter efi'ect is wellknown to those skilled in the art. It is, therefore, to be understoodthat I do not wish to be limited to any given number of stages ofamplification or by any fixed arrangement of constants.

In the arrangement shown in Figure 8, a different embodiment'of upperthreshold network is shown, employing a tube with two anodes. In thisarrangement, when a very large voltage occurs at the rectifier (as forexample that occasioned by the first arrival or direct wave) therectifier will pass current and deliver this to the filter network. Thetube 22!, by means of its two anodes 222 and 223, will bypass allpotential above that determined by the threshold and particularly by theadjustment of arm 224 to the potential of battery 225. It will beobserved that the arm 224 and the battery 225 in conjunction with tube22! furnish the means of providing a setting of a limit above which itis impossible for the governing floating charge to go. It may be well topoint out at this time that a positive floating charge is used.

Similarly, the adjustment of arm 230 upon resistance 23! in connectionwith battery 232 provides a means for adjusting the lower threshold.

From the foregoing, it will be seen that I have accomplished the objectsof my invention. Without theamplitude limiting arrangement practicallyall of the amplitudes received except those from very deep lying strataare too large for analysis. Amplitudes decay and die out very rapidly atthe end of the record and only a veryshort interval exists where properanalysis can.

be made. With the use of an amplitude limiting system such as shown inFigure 2, certain disadtions from strata at diiferent depths. A completeobservation and usable record can be made from. a single shot by the useof the ethod of my invention.

. It will be understood that certain features and sub-combinations areof utility-and may be employed without reference to other features andsub-combinations. This is contemplated by and is within the scope of myclaims. It is further obvious that various changes may be made indetails within the scope of my claims without departing from the spiritof my invention. It is, therefore, to be understood that my invention isnot to be limited to the specific details shown and described.

Having thus claim is:

1. In a method of making geophysical explora-' tions in which earthvibrations are generated, the vibrations and reflections thereof fromgeological strata are converted into electromotive forces oi varyingvoltages in sympathy with said vibrations and reflections, and saidvoltages are amplified and recorded, the stepsof amplifying voltagesbeiow a predetermined value a predetermined amount, amplifying voltagesbetween said predetermined value and a second predetermined valueinversely as a function of the voltage, and amplifying voltages higherthan said second predetermined value an amount equal to theamplidescribed my invention, what I fication voltages. of said secondpredetermined value.

2. Apparatus for making geophysical explorations including incombination a seismophone adapted to convert earth vibrations intoelectromotive forces of varying voltages in sympathy with saidvibrations, an amplification channel, a recorder, means for impressingthe output of said seismophone upon said amplification channel, andmeans responsive to the output of said amplification channel betweenpredetermined voltages for varying the sensitivity of said amplificationchannel inversely as a function of the voltage of said output.

3. Apparatus as in claim 2 including means for amplifying voltages belowthe lower of said predetermined voltages at the maximum sensitivity ofsaid amplification channel.

4. Apparatus as in claim 2, including means for amplifying voltagesabove the upper of said predetermined voltages at the sensitivity atwhich the amplification channel amplifies said upper voltage. I Y

5. In a method of making geophysical explorations in which earthvibrations are generated, the vibrations converted into electromotiveforces of varying voltages in sympathy with said vibrations, the stepsof amplifying the voltages and varying the degree of amplification ofvoltages between predetermined values inversely as a function of thevoltage.

6. An apparatus for making geophysical explorations including incombination a seismophone, an amplifier having a thermionic tube, arecorder,

means for impressing the output of said seismophone upon said amplifier,means for delivering a portion of the output of said amplifier to saidrecorder, a thermionic valve, a condenser, means for impressing aportion ofthe output of said amplifier upon said thermionic valve, meansfor delivering the output of said thermionic valve to said condenser,means for biasing said thermionic valve to prevent it from passingcurrent below a predetermined voltage, and means for impressing thecharge of said condenser upon the grid-of the thermionic tube of saidamplifier.

7. In an apparatus for making geophysical expiorations having incombination a seismophone. an amplifier having a thermionic tube, arecorder, means for impressing the output of said .seismophone upon saidamplifier, and means for delivering a portion 01 the output of saidamplifier to said recorder; a first thermionic valve, a

condenser, means for impressing a portion of the.

output of said amplifier upon said first thermionic valve, means fordelivering the output of said first thermionic valve to said condenser,means for impressing the charge of said condenser upon the grid of saidthermionic tube of said amplifier, a second thermionic valve bypassingsaid condenser, and means for biasing said second thermionic vaivetopermit said condenser to discharge voltages above a predeterminedamount.

8. An apparatus for making geophysical explorations including incombination a seismophone, an amplifier having a thermionic tube, meansfor impressing the' output of said seismophone upon said amplifier, afirst thermionic valve, a condenser, means for impressing a portion ofthe output of said amplifier upon said first thermionic valve, means fordelivering the output of said first thermionic valve to said con-- minedvoltage.

HAROLD R. PRESCOTT.

